This invention relates to a method and apparatus for treating a combustible gas stream containing hydrogen sulphide.
Hydrogen sulphide containing gas streams (sometimes referred to as `acid gas streams`) are typically formed in oil refineries and natural gas processing units. Such streams cannot be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the residual sulphur dioxide takes place. Resulting sulphur vapour is extracted downstream of each such stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, e.g. by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere.
Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is of course present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of given size. This ceiling can be raised by using commercially pure oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.
Most Claus plants are equipped with right cylindrical furnaces having a length to internal diameter ratio in the range of from two to four. The furnaces are typically cross-fired or tangentially-fired by a burner or burners mounted at the side. Cross or tangentially fired burners achieve good mixing of the reacting chemical species. If desired, mixing can be enhanced by equipping the furnace with baffles or checkerwork walls.
A particular perceived disadvantage of such cross or tangentially fired arrangements is that if commercially pure oxygen or oxygen-enriched air is used to support the combustion of the hydrogen sulphide there is a relatively high risk of damage to the refractory lining of the furnace being created by the resulting increase in flame temperature. There are a number of proposals in the art to solve this problem. Some proposals involve introduction of flame moderators such as water into the furnace; others involve recycling to the furnace gas from a downstream part of the plant so as to moderate the temperature in the furnace; and yet others employ a plurality of furnaces so as to limit the amount of combustion that is performed in each individual furnace, thereby obviating the need for an external flame moderator or to recycle gas from a downstream part of the furnace. All these proposals, however, add to the complexity of the plant.
One way of reducing the potential for damage to the refractory lining when commercially pure oxygen or oxygen-enriched air is used to support combustion of hydrogen sulphide is to employ axially or longitudinally fired burners mounted on the back wall instead of cross or tangentially fired burners mounted at the side of the furnace. Such axially or longitudinally fired burners have been shown to give average residence times comparable with those of cross or tangentially fired burners (typically from 1 to 2 seconds) at design throughput.
The use of such an axially or longitudinally fired burner is disclosed in European patent application 0 315 225A in which there is a central pipe for oxygen, at least one second pipe for hydrogen sulphide containing feed gas which coaxially surrounds the central pipe, and an external coaxial pipe for air. The burner is used when the hydrogen sulphide feed gas contains at least 5% by volume of hydrocarbons or carbon dioxide. The oxygen velocity at the outlet of the burner is in the range of from 50 to 250 metres per second and the corresponding feed gas velocity is in the range of 10 to 30 metres per second. Temperatures in the range of from 2000 to 3000.degree. C. are generated in the core of the burner flame, and a gas mixture having a temperature in the range of 1350 to 1650.degree. C. leaves the furnace. This gas mixture contains at least 2% by volume of carbon monoxide and at least 8% by volume of hydrogen.
WO-A-96/26157 also discloses the use of an axially or longitudinally fired burner in the Claus process. Generally parallel flows of a first gas containing hydrogen sulphide and a second gas enriched in oxygen are supplied to the tip of the burner. The ratio of the velocity of the first gas to the velocity of the second gas is selected so as to be in the range of from 0.8:1 to 1.2:1. In typical examples the burner fires longitudinally into a furnace having a length of 8 m and a diameter of 1.5 m. The diameter of the burner is 0.4 m.
We have discovered that a problem arises when an axially or longitudinally fired burner is employed in a Claus furnace. This problem is that considerable short circuiting of gas molecules from the burner tip to the furnace exit occurs. Moreover, the provision of baffles or checkerwork walls within the furnace has little effect in reducing this short-circuiting.
It is an aim of the present invention to provide a method of and an apparatus for treating a combustible gas stream containing hydrogen sulphide which solve or ameliorate this problem. Other aims of the invention will become apparent from the ensuing description.